1. Field of the Invention
The invention relates to particles having zwitterionic structural elements on the surface, and also their production and use, in particular in composite materials.
2. Description of the Related Art
Finely divided particles which, when added to a matrix alter its properties, are referred to as fillers. Fillers are nowadays used for many purposes in the chemical industry. They can alter the mechanical properties of plastics, e.g. hardness, tear strength, resistance to chemicals, electrical, thermal and ionic conductivities, adhesion or coefficient of thermal expansion. Surface modification of the fillers generally allows the compatibility of the filler with the matrix to be influenced and the property profile of the composite to be significantly improved as a result. Preference is given to using modified fillers which are reactive toward the polymer matrix. Thus, for example, carbinol-functional particles can be covalently bound to an isocyanate-functional resin. Such chemical incorporation of a filler into the polymer matrix is frequently associated with a considerable improvement in properties, for example mechanical reinforcement. A comparable mechanical reinforcement of a polymer is observed when a polar or ionic filler is incorporated into an appropriate polar or ionic matrix. In this case, the mechanical reinforcement is achieved by means of purely physical interactions (ionic interactions, dipole-dipole interactions) between filler and matrix.
Finely divided particles which when added to a matrix alter its properties are referred to as fillers. Fillers are nowadays used for many purposes in the chemical industry. They can alter the mechanical properties of plastics, e.g. hardness, tear strength, resistance to chemicals, electrical, thermal and ionic conductivities, adhesion or coefficient of thermal expansion. Surface modification of the fillers generally allows the compatibility of the filler with the matrix to be influenced and the property profile of the composite to be significantly improved as a result. Preference is given to using modified fillers which are reactive toward the polymer matrix. Thus, for example, carbinol-functional particles can be covalently bound to an isocyanate-functional resin. Such chemical incorporation of a filler into the polymer matrix is frequently associated with a considerable improvement in properties, for example mechanical reinforcement. A comparable mechanical reinforcement of a polymer is observed when a polar or ionic filler is incorporated into an appropriate polar or ionic matrix. In this case, the mechanical reinforcement is achieved by means of purely physical interactions (ionic interactions, dipole-dipole interactions) between filler and matrix.
Customary proton-conducting polymer electrolyte membranes (PEMs) as are used, for example, for producing fuel cells are made up of polymers which have covalently bound proton-conducting groups, in particular sulfonic acid groups or amino groups.
A problem which frequently occurs when the polymer electrolyte membranes are used in fuel cells is the low mechanical stability of the membranes. However, a high mechanical stability of the membranes against, inter alia, the pressure of the gases used as fuel is absolutely necessary for industrial use of the fuel cells. Particularly when polymer electrolyte membranes are used in fuel cells for automobile applications, membrane materials which can withstand very high mechanical stresses are necessary.
A particularly advantageous way of solving this problem is to use fillers which are capable of mechanically reinforcing the polymer matrix. If the fillers additionally have proton-conducting functions on their surface or the fillers used are materials having intrinsic conductivity, an improved proton conductivity of the composite membrane can be expected in addition to an improvement in the mechanical stability. Furthermore, the undesirable permeability for the fuel can be suppressed when suitable fillers are used.
Such composite membranes comprising proton-conducting polymers and inorganic particles are known from the literature. Thus, the documents US 2004/0053060, U.S. Pat. No. 5,919,583, WO 0045447, US 2005/0053818 and JP 2001/155744 describe mixtures comprising a proton-conducting organic polymer and unmodified or modified inorganic fillers. Fillers used are, inter alia, unmodified silica sols, aminoalkyl-functional silica sols and silica sols having sulfonic acid functions, and organic polymers used are sulfonated perfluorinated hydrocarbons and sulfonated polyether ketones.
The property profile of the membrane can in actual fact be improved appreciably by incorporation of such particles. However, optimal results are not achieved in all systems described in the prior art. In particular, the corresponding composite membranes still have an unsatisfactory proton conductivity and a mechanical stability which is insufficient for industrial use, as indicated, inter alia, by an insufficient modulus of elasticity and an unsatisfactory tensile strength.
A possible way of improving these properties is to use fillers which bear zwitterionic structural elements in which cationic and anionic groups are covalently bound to one another. Such fillers and their use for producing composite materials are not described in the literature.
On the other hand, silanes and siloxanes having zwitterionic structural elements, which can be obtained, for example, by reacting an aminosilane or an amino-functional siloxane with a halogenated carboxylic acid or an alkyl sultone, are known from the literature. In J. APPL. POLYM. SCI. 1975, 19, 1221-1225, Litt et al. teach the preparation of two zwitterionic alkoxysilanes by reaction of aminopropyltriethoxysilane or N-aminoethylaminopropyltrimethoxysilane with 1,3-propane sultone.
The documents DE 3 417 912 and DE 3 422 268 describe zwitterionic organofunctional siloxanes which can be obtained by quaternization of aminoalkyl-substituted polydimethylsiloxanes by means of ω-haloalkyl-carboxylates.
Zwitterionic polysiloxanes obtained by reaction of ethylenediamino-functional polysiloxanes with 1,3-propane sultone are described by Graiver et al., in J. POLYMER SCI. 1979, 17, 3559-3572.
In U.S. Pat. No. 4,918,210 and in LANGMUIR 1990, 6,385-391, Snow et al. teach the preparation of polysiloxanes containing quaternary ammonioalkylsulfonate groups and their use as a class of surface-active substances by means of which the surface tension of aqueous solutions can be considerably reduced. The silicone sulfobetaines reduce the surface tension at significantly lower reagent concentrations than the corresponding functionalized organic agents. U.S. Pat. No. 4,496,795 describes such zwitterionic siloxanes, which are occasionally referred to as silicone sulfobetaines in the literature, as thermoplastic elastomers which due to their good adhesion to wood, metal, polycarbonate and polystyrene are suitable as, inter alia, sealants and adhesives and for coatings. The thermoplastic properties and in particular the excellent mechanical properties of the zwitterionic siloxanes are attributed to ionic crosslinking of adjacent zwitterionic siloxane chains.